Interspecific variation in photo-induced GnRH plasticity among nomadic cardueline finches.

Changes in response to photoperiod are of fundamental importance to seasonal cycles in avian reproductive development. In this study we present data on photo-induced changes in gonadotropin-releasing hormone (GnRH) immunoreactivity associated with the development of photorefractoriness in males of 3 closely related species of cardueline finch: common redpoll (Carduelis flammea), pine siskin (C. pinus) and white-winged crossbill (Loxia leucoptera). All are nomadic, mid- to high-latitude breeders with varying levels of reproductive flexibility: redpolls are typically seasonal, pine siskins more flexibly seasonal and white-winged crossbills temporally opportunistic. Males were exposed to either long (20L:4D) or short days (5L:19D), beginning in January, and GnRH was assessed via immunocytochemistry 4.5 months later. Plasma LH, testis size, and molt score were also measured. Significant reductions in the number of irGnRH cells detected, cross-sectional cell area and optical density of these cells occurred in both redpolls and siskins on long, as compared with short, days. These decreases in irGnRH, coupled with spontaneous testicular regression and onset of molt were consistent with the development of absolute photorefractoriness on constant long days. In contrast, the tendency for numbers of GnRH immunopositive cells and mean measures of GnRH immunoreactivity (cell area, optical density and axonal fiber density) to decrease were much more moderate in white-winged crossbills held on long versus short days. Although none of the four measures of GnRH immunoreactivity differed significantly between treatments in crossbills, significant shifts in the frequency distribution of irGnRH neurons to include greater numbers of less immunoreactive cells suggested that antigenic GnRH content was reduced in many cells. These photo-induced shifts in optical densities of GnRH cells in long-day, as compared with short-day crossbills followed, to a lesser extent, the pattern in pine siskins and common redpolls. For all three species, patterns of decrease in GnRH immunoreactivity, gonadal regression and molt suggest that some form of refractoriness to photostimulation develops on constant long days. However, more moderate tendencies for GnRH immunoreactivity to decrease in white-winged crossbills, coupled with limited gonadal regression and molt on long days suggest that absolute photorefractoriness might develop more slowly in this taxon than in others, or alternatively, a condition arises that is more similar to relative photorefractoriness. These data indicate that relatively small differences in the response of the neuroendocrine system to stimulatory cues such as day length may lead to remarkably different capabilities for reproductive flexibility even within a closely related family of birds.

[Cranial irradiation induces premature activation of the gonadotropin-releasing-hormone]. / Schädelbestrahlung verursacht vorzeitige Aktivierung des Gonadotropin-Releasinghormon (GnRH)-Pulsgenerators bei Ratten - Ein neuesTiermodell für strahleninduzierte Störungen der Pubertät.

BACKGROUND: CNS-irradiation in prepubertal children with leukemia or brain tumors can lead to precocious or in high doses to delayed puberty. The underlying mechanisms of these disorders are unknown. METHODS: A new animal model of experimentally induced pubertal disorders by cranial irradiation has been developed. In infantile or juvenile (12 - 23 days old) female rats precocious or delayed puberty have been induced by selective cranial Co60-irradiation (4 - 18 Gy). At age of 32 - 38 days or 3 months relevant hormone parameters have been studied basal and after stimulated conditions. RESULTS: Low radiation doses (5 or 6 Gy) led to accelerated onset of puberty as well as elevated LH- and estradiol levels. High radiation doses (9 - 18 Gy) caused retardation of sexual development, lower gonadotropin levels and growth retardation associated with growth hormone deficiency. After cranial irradiation with 5 Gy the release rates of the inhibitory neurotransmitter gamma-aminobutyric-acid (GABA) from hypothalamic explants were significantly lower (p < 0,05). The gonadotropin-releasing-hormone (GnRH) expression in the hypothalamic preoptic area of irradiated animals (5 Gy) was significantly higher than in controls (p < 0,05). CONCLUSION: The GnRH-pulse generator is very radiosensitive as low dose irradiation causes precocious puberty, whereas high dose irradiation is associated with delayed sexual maturation. Radiation induced precocious puberty might be caused by damage to inhibitory GABAergic neurons leading to desinhibition and premature activation of GnRH neurons. Our animal model of cranial irradiation seems to be suitable to study neurotransmitter disorders, molecular mechanisms and potential preventive intervention of radiation induced pubertal changes.

Reduction of gamma-aminobutyric acid-ergic neurotransmission as a putative mechanism of radiation induced activation of the gonadotropin releasing-hormone-pulse generator leading to precocious puberty in female rats.

Brain irradiation in prepubertal children with malignomas can cause precocious puberty. A selective cranial cobalt (Co(60))-irradiation technique has been developed in rats. In two experiments early juvenile (13-15 days old) female rats received a single dose of 5 Gy or sham irradiation. At pubertal age (post-natal days 33-34) irradiated rats had higher serum estradiol and luteinizing hormone levels. In experiment 1 irradiated rats had higher gonadotropin releasing-hormone (GnRH) mRNA levels in the preoptic area compared to controls (P<0.05). In experiment 2 the release rates of gamma-aminobutyric acid (GABA) in vitro from preoptic mediobasal hypothalamic areas of irradiated rats were significantly reduced after stimulation with the GABA(A) receptor agonist muscimol (maximum values 4607+/-804 vs. 7399+/-1048 pM in controls, mean+/-SEM, P<0.05). Radiation induced central precocious puberty might be caused by damage to inhibitory GABAergic neurons leading to premature activation of the GnRH-pulse generator.

Seasonal variation in brain GnRH in free-living breeding and photorefractory house finches (Carpodacus mexicanus).

Absolute photorefractoriness in captive birds of several species correlates with reduced hypothalamic content of gonadotropin-releasing hormone (GnRH) relative to photosensitive or photostimulated birds. We used immunocytochemistry to examine GnRH immunostaining in free-living breeding and photorefractory house finches (Carpodacus mexicanus). Photorefractory birds with regressed gonads in early autumn had lower GnRH immunoreactivity (fewer, smaller, less intensely stained cell bodies and fewer immunopositive axon fibers) than did breeding birds with enlarged gonads in spring. These results confirm that absolute refractoriness in house finches is associated with dramatic changes in the hypothalamic GnRH system, as in other species with this type of refractoriness, and show that these changes occur in free-living birds.